An investigation of the flow regimes resulting from splashing drops

Numerical simulations have been used to investigate the flow regimes result ing from the impact of a 2.9 mm water drop on a deep water pool at velociti es in the range 0.8-2.5 m/s. The results were used to identify the conditio ns leading to the formation of vortex rings, entrapment of a bubble during cavity collapse and the formation of vertical Rayleigh jets. Bubble entrapm ent and the associated growth of a thin high speed jet were shown to be the result of a capillary wave that propagates down the walls of the crater re sulting from drop impact. Although the existence of a capillary waves is a necessary condition for bubble entrapment, bubbles will only occur when the wave speed and maximum crater size is such that the wave reaches the botto m of the crater before collapse has resulted in the formation of a thick Ra yleigh jet. Simulations also clarified the conditions for which drop impact leads to axi-symmetric vortex rings. Results not reported previously, incl ude the observation that a single drop can produce multiple vortex rings an d that vortex rings can occur for conditions that lead to broad Rayleigh je ts. Based on these results, it was concluded that the formation of vortex r ings depends on the time at which vorticity is generated and the nature of its subsequent transport. (C) 2000 American Institute of Physics. [S1070-66 31(00)01404-5].